Re: [PATCH 19/24] rcu/tree: Support reclaim for head-less object

Paul E. McKenney
2020-05-01T22:39:11Z

On Tue, Apr 28, 2020 at 10:58:58PM +0200, Uladzislau Rezki (Sony) wrote:
> Update the kvfree_call_rcu() with head-less support, it
> means an object without any rcu_head structure can be
> reclaimed after GP.
>
> To store pointers there are two chain-arrays maintained
> one for SLAB and another one is for vmalloc. Both types
> of objects(head-less variant and regular one) are placed
> there based on the type.
>
> It can be that maintaining of arrays becomes impossible
> due to high memory pressure. For such reason there is an
> emergency path. In that case objects with rcu_head inside
> are just queued building one way list. Later on that list
> is drained.
>
> As for head-less variant. Such objects do not have any
> rcu_head helper inside. Thus it is dynamically attached.
> As a result an object consists of back-pointer and regular
> rcu_head. It implies that emergency path can detect such
> object type, therefore they are tagged. So a back-pointer
> could be freed as well as dynamically attached wrapper.
>
> Even though such approach requires dynamic memory it needs
> only sizeof(unsigned long *) + sizeof(struct rcu_head) bytes,
> thus SLAB is used to obtain it. Finally if attaching of the
> rcu_head and queuing get failed, the current context has
> to follow might_sleep() annotation, thus below steps could
> be applied:
> a) wait until a grace period has elapsed;
> b) direct inlining of the kvfree() call.
>
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> kernel/rcu/tree.c | 102 ++++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 98 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 51726e4c3b4d..501cac02146d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3072,15 +3072,31 @@ static void kfree_rcu_work(struct work_struct *work)
> */
> for (; head; head = next) {
> unsigned long offset = (unsigned long)head->func;
> - void *ptr = (void *)head - offset;
> + bool headless;
> + void *ptr;
>
> next = head->next;
> +
> + /* We tag the headless object, if so adjust offset. */
> + headless = (((unsigned long) head - offset) & BIT(0));
> + if (headless)
> + offset -= 1;
> +
> + ptr = (void *) head - offset;
> +
> debug_rcu_head_unqueue((struct rcu_head *)ptr);
> rcu_lock_acquire(&rcu_callback_map);
> trace_rcu_invoke_kvfree_callback(rcu_state.name, head, offset);
>
> - if (!WARN_ON_ONCE(!__is_kvfree_rcu_offset(offset)))
> + if (!WARN_ON_ONCE(!__is_kvfree_rcu_offset(offset))) {
> + /*
> + * If headless free the back-pointer first.
> + */
> + if (headless)
> + kvfree((void *) *((unsigned long *) ptr));
> +
> kvfree(ptr);
> + }
>
> rcu_lock_release(&rcu_callback_map);
> cond_resched_tasks_rcu_qs();
> @@ -3221,6 +3237,13 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> if (IS_ENABLED(CONFIG_PREEMPT_RT))
> return false;
>
> + /*
> + * TODO: For one argument of kvfree_rcu() we can
> + * drop the lock and get the page in sleepable
> + * context. That would allow to maintain an array
> + * for the CONFIG_PREEMPT_RT as well. Thus we could
> + * get rid of dynamic rcu_head attaching code.
> + */
> bnode = (struct kvfree_rcu_bulk_data *)
> __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> }
> @@ -3244,6 +3267,23 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> return true;
> }
>
> +static inline struct rcu_head *
> +attach_rcu_head_to_object(void *obj)
> +{
> + unsigned long *ptr;
> +
> + ptr = kmalloc(sizeof(unsigned long *) +
> + sizeof(struct rcu_head), GFP_NOWAIT |
> + __GFP_RECLAIM | /* can do direct reclaim. */
> + __GFP_NORETRY | /* only lightweight one. */
> + __GFP_NOWARN); /* no failure reports. */

Again, let's please not do this single-pointer-sized allocation. If
a full page is not available and this is a single-argument kfree_rcu(),
just call synchronize_rcu() and then free the object directly.

It should not be -that- hard to adjust locking for CONFIG_PREEMPT_RT!
For example, have some kind of reservation protocol so that a task
that drops the lock can retry the page allocation and be sure of having
a place to put it. This might entail making CONFIG_PREEMPT_RT reserve
more pages per CPU. Or maybe that would not be necessary.

Thanx, Paul

> + if (!ptr)
> + return NULL;
> +
> + ptr[0] = (unsigned long) obj;
> + return ((struct rcu_head *) ++ptr);
> +}
> +
> /*
> * Queue a request for lazy invocation of appropriate free routine after a
> * grace period. Please note there are three paths are maintained, two are the
> @@ -3260,16 +3300,34 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> {
> unsigned long flags;
> struct kfree_rcu_cpu *krcp;
> + bool success;
> void *ptr;
>
> + if (head) {
> + ptr = (void *) head - (unsigned long) func;
> + } else {
> + /*
> + * Please note there is a limitation for the head-less
> + * variant, that is why there is a clear rule for such
> + * objects:
> + *
> + * it can be used from might_sleep() context only. For
> + * other places please embed an rcu_head to your data.
> + */
> + might_sleep();
> + ptr = (unsigned long *) func;
> + }
> +
> krcp = krc_this_cpu_lock(&flags);
> - ptr = (void *)head - (unsigned long)func;
>
> /* Queue the object but don't yet schedule the batch. */
> if (debug_rcu_head_queue(ptr)) {
> /* Probable double kfree_rcu(), just leak. */
> WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
> __func__, head);
> +
> + /* Mark as success and leave. */
> + success = true;
> goto unlock_return;
> }
>
> @@ -3277,10 +3335,34 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> * Under high memory pressure GFP_NOWAIT can fail,
> * in that case the emergency path is maintained.
> */
> - if (unlikely(!kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr))) {
> + success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
> + if (!success) {
> + if (head == NULL) {
> + /*
> + * Headless(one argument kvfree_rcu()) can sleep.
> + * Drop the lock and tack it back. So it can do
> + * direct lightweight reclaim.
> + */
> + krc_this_cpu_unlock(krcp, flags);
> + head = attach_rcu_head_to_object(ptr);
> + krcp = krc_this_cpu_lock(&flags);
> +
> + if (head == NULL)
> + goto unlock_return;
> +
> + /*
> + * Tag the headless object. Such objects have a
> + * back-pointer to the original allocated memory,
> + * that has to be freed as well as dynamically
> + * attached wrapper/head.
> + */
> + func = (rcu_callback_t) (sizeof(unsigned long *) + 1);
> + }
> +
> head->func = func;
> head->next = krcp->head;
> krcp->head = head;
> + success = true;
> }
>
> WRITE_ONCE(krcp->count, krcp->count + 1);
> @@ -3294,6 +3376,18 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>
> unlock_return:
> krc_this_cpu_unlock(krcp, flags);
> +
> + /*
> + * High memory pressure, so inline kvfree() after
> + * synchronize_rcu(). We can do it from might_sleep()
> + * context only, so the current CPU can pass the QS
> + * state.
> + */
> + if (!success) {
> + debug_rcu_head_unqueue(ptr);
> + synchronize_rcu();
> + kvfree(ptr);
> + }
> }
> EXPORT_SYMBOL_GPL(kvfree_call_rcu);
>
> --
> 2.20.1
>